ByRobert C. CowenJune 2, 1982

Dust from Mexico's El Chichon volcano has joined an earlier "mystery cloud" to spread a thin veil in the stratosphere of the Northern Hemisphere. And as it spreads, so does speculation about its possible effect on weather.

Happily, no meteorological disaster need be expected. While the spreading dust may cause some colorful sunsets, its most likely weather influence would be a drop in average Northern Hemisphere temperature of 0.3 to 0.5 degrees C.

Climatologist Brian Toon of the NASA-Ames Research Center at Moffett Field, Calif., who made this estimate, says it is possible that some locations might have a shortened growing season or cooler average temperature. On the other hand , he adds, it's quite possible there will be no detectable temperature change at all.

That's a much more modest outlook than the 1 to 10 degree C. cooling foretold by one TV network newscaster. With that much cooling, you might well expect to see the glaciers of a new ice age. Such exaggerated speculation about climatic effects of volcanic dust was also heard when Mt. St. Helens began erupting. But there is no evidence at all of its having had any remarkable weather effect.

Such alarms about imminent climatic disaster raise needless fear even though, over the long term, volcanic dust may indeed have an important climatic influence.

A number of climatologists have linked periods of substantially increased volcanic activity with climatic cooling. Even unusually severe individual eruptions can sometimes be linked to weather. The erupton of Mt. Tambora in Indonesia in 1816, for example, correlates with the "year without a summer," when severe weather caused widespread crop failures in parts of the Northern Hemisphere. Some climatologists see a relationship, suggesting that volcanic dust cuts the amount of sunshine reaching the lower atmosphere and thus leads to cooling. But this is not yet a universally accepted theory.

Even if such a cause-effect relationship were certain, the present dust cloud is considered too small to have much influence. The Mauna Loa (Hawaii) Observatory of the US National Oceanic and Atmospheric Administration first picked up the El Chichon cloud on April 9. Since then, it has been monitored by ground observations and by NASA-Ames aircraft. It appears to contain about 10 million tons of sulfuric acid, the main constituent of most volcanic clouds. That's about ten times as much as the 1980 Mt. St. Helens cloud. It also appears to have much more material than the "mystery cloud" first observed last winter, which is attributed to an unidentified volcano.

Spreading through the stratosphere at heights between about 18 and 27 kilometers, the dust is distributed in several layers that are gradually spreading. Some travel eastward on westerly winds. Other layers, suspended in easterly air flows, move west.

This should color may sunsets by scattering sunshine. It will also give meteorologists a valuable opportunity to look for possible, and subtle, weather effects. But any concern that it will bring on a deep feeeze can be discounted.